1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to curable organosiloxane compositions containing finely divided silver particles that impart electrical conductance to cured materials prepared from these compositions. The silver particles contain a unique coating that not only increases the electrical conductivity of cured materials containing these particles relative to the values achieved using silver particles coated with known treating agents but also improves the ability of the materials to retain this conductivity during long term exposure to temperatures above 100.degree. C.
2. Background Information
It is known to impart electrical conductivity to a variety of organic and organosilicon polymers by adding a finely divided electrically conductive material such as carbon black or silver to the polymer composition. It is also known to treat silver with organic fatty acids such as stearic acid to facilitate processing of the silver to form finely divided particles and blending large concentrations of the resultant particles into polymer compositions.
Japanese patent publication No. 60/1222, published on Jan. 7, 1985 describes electrically conductive pastes prepared by blending a liquid epoxide resin, dicyandiamide curing accelerators and reactive diluents with a particulate form of silver. The exemplified reactive diluent is the glycidyl ester of a fatty acid containing 14 carbon atoms.
Japanese Laid-Open Application No. 01/31874 teaches using a mixture containing 70 to 95 weight percent of silver flake having a diameter of from 1 to 10 microns and from 30 to 5 weight percent of a spherical silver powder having a diameter of from 0.1 to 0.5 micron together with finely divided silica in a conductive epoxy resin paste.
U.S. Pat. No. 4,695,404, issued on Sep. 22, 1987 describes polymeric compositions having a volume resistivity no higher than 0.1 milliohm-cm. The compositions comprise a polymer and from 70 to 90 weight percent, based on the weight of the composition, of silver flakes exhibiting a volume resistivity in the compressed state of no more than 0.2 milliohm-cm. The polymer can be an epoxy resin, silicone, fluorosilicone, polyurethane or long chain hydrocarbon.
Other polymer compositions containing silver as a conductive filler are described in published European Patent Application No. 367,562, published on May 9, 1990; European Patent Application No. 388,473; Japanese Laid Open Application No. 1/189,806; Japanese Laid Open Patent Application No. 1/153,767; Japanese Laid Open Patent Application No. 1/31,874; Japanese Laid Open Patent Application No. 63/161014; and U.S. Pat. No. 3,685,028.
Because solid silver particles typically provide higher electrical conductivity than an equal weight of silver coated particulates such as glass beads and fibers, the solid particles are usually preferred when it is desired to maximize electrical conductivity.
Finely divided solid silver particles are available as a spherical powder and in flake form. Because the interparticle contact area of flakes is typically higher than that of powders, flakes are preferred in some applications.
Commercially available forms of silver flake typically contain a coating of a fatty acid as a processing aid. If the coating is not removed while the flake is being incorporated into a composition, the presence of the coating on the silver flake will reduce the electrical conductivity of the final composition. Because the fatty acid is polar and curable organosiloxane compositions are typically non-polar, the fatty acid coating tends to remain on the particles during preparation of the compositions and adversely affects the electrical conductivity of the cured material. The presence of the fatty acid may also interfere with curing of the organosiloxane composition.
While spherical silver particles without any coating are commercially available, these are difficult to disperse in organosiloxane compositions at the concentration required to achieve volume resistivity values less than 1 milliohm/cm following heat aging of the cured sample at temperatures of about 150 for at least two days.
An objective of this invention is to increase the electrical conductivity of elastomers, gels and resins prepared from curable organosiloxane compositions by increasing the concentration of finely divided silver particles that can be incorporated into the composition without increasing the viscosity of the composition to the extent that it is no longer suitable for the intended end use application, for example as a coating material or adhesive.
A second objective of the present invention is to provide silver-filled curable organosiloxane compositions that cure to yield electrically conductive gels, elastomers and resins which retain their initial level of electrical conductivity following long term exposure to elevated temperatures, typically 150.degree. C. and above.